Although the positive association between body weight and blood pressure (BP) is well documented, fatness per se may not be the direct, causal factor accounting for the elevated BP level found in many obese individuals. Recognizing the complexity of predisposing factors for development of hypertension, we have developed a multivariate step-wise regression model to analyze the relative contribution of fatness to BP level. The results suggest that body fatness (absolute and percentage) is not a significant predictive factor of BP level relative to lean body mass, age, body build, and an upper body fat pattern. These findings led to the hypothesis that the fall in BP observed with weight loss may be due to an adaptive response to energy deprivation rather than loss of fat per se. BP, hormone, electyrolyte and hemodynamic status of obese adults with borderline hypertension will be measured 1) during energy balance prior to fat loss, 2) during acute energy restriction, and 3) during energy balance at reduced body fat. Sodium, potassium, calcium, alcohol and the percent of energy derived from carbohydrate, protein and fat will be controlled to avoid the confounding effects of concurrent dietary modifications. The protocol will consist of 4 weeks of hospitalization initially, followed by 2-3 months of out-patient monitoring, and 1 week of final hospitalization. The results of the proposed study should serve to expand our understanding of the etiology of hypertension and the role of weight loss as a therapeutic modality for treatment of mild-moderate hypertension.